The mentioned parallel surfaces are understood as the surfaces of the waveguide also being parallel to the surface of the substrate to which the thin film waveguide is arranged. These parallel surfaces of the thin film waveguide form interfaces between the waveguide and air or a capping and between the waveguide and the substrate.
Concentrating light in the context of the invention happens at least by collecting light using a thin film waveguide having two parallel surfaces for coupling light into the thin film waveguide through at least one of these surfaces.
A dielectric thin film waveguide as known in the art is typically arranged, preferably by material deposition, as a stack of transparent thin films of an overall thickness of 300 nm to 1 pm or only a few pm on a supporting substrate. One surface of the thin film waveguide contacts the substrate. One other surface of the thin film waveguide contacts the surrounding air or a capping. At least one of these two parallel surfaces forms a surface of incidence through which light may be coupled into the thin film waveguide.
With the thin film waveguide thickness in the range of a few hundred nanometers to a few micrometers only and the diameter of a propagating mode being in the same order the collecting area at the surface of the thin film waveguide will typically exceed the cross section of the excited waveguide modes.
In that sense the collection of light into guided modes means concentrating the light, particularly in a first step.
In a possible second step termed planar concentration the guided modes may be further concentrated, for instance by reducing the waveguides cross section, particularly the width and/or tapering it into stripe waveguides, which further reduces the modal cross section.
Propagation of a guided mode in the thin film waveguide takes place—if not restricted to one specific direction or several specific directions by other means—in all directions being parallel to the surface of the thin film waveguide and perpendicular to the normal vector of the substrate surface. These directions are understood as lateral. Accordingly the excited guide modes are also named as lateral modes.
Collecting light through the surface of the thin film waveguide may take place in any direction that leads to a refraction of the light incident to the surface into the volume of the thin film waveguide.
In a perfectly planar passive dielectric thin film waveguide with smooth parallel surfaces an excitation of guided lateral modes from the outside through one of these surfaces is impossible (Snell's law). In fact this can be accomplished by braking the planarity with refractive index variations, backside scattering or diffractive elements. However, these concepts suffer from a symmetry between excitation and extraction of guided waveguide modes. In other words in the state-of-the-art passive planar thin film waveguides all measures taken to enable coupling light into the waveguide via at least one of its surfaces also lead to improved extraction of the trapped light through the same surface. Accordingly only a small amount of the trapped light is guided in the thin film waveguide, for example by the principle of total reflection in a film mode excited in the waveguide.
The state of the art luminescent solar concentrators (LSCs) overcome this disadvantage of passive waveguides. In such “active” concentrators photons being coupled into the waveguide are absorbed in a specific range of wavelength by means of a luminescent waveguide medium and emitted by the principle of luminescence into planar waveguide modes at a longer wavelength. Accordingly in principle the method of coupling light into the waveguide by absorption and emission does not affect the traveling of the emitted waveguide modes due to the different wavelength and the stokes-shift of absorption and emission spectrum of the used medium.
Typically such luminescent waveguides are plates of a few millimeters in thickness that have two parallel surfaces being spaced by their edge surfaces that connect the two parallel surfaces in the direction of their respective normal vectors. The spacing/distance between the two parallel surfaces is small (typically at least 10 times smaller) compared to the extension of the collecting area in both dimensions. The sum of all edge surface areas that may be used for extraction (extraction area) of the concentrated light is less than the thin film waveguide surface area used for coupling light into the waveguide (collection area).
Solar cells may be arranged at these edges of such an LSC to convert the guided light energy into electricity. The concentration factor of LSCs, defined as the ratio of the collection area to the extraction area is typically limited to about 10. This is firstly due to the large thickness of the waveguide and secondly due to the collecting area that is typically limited to tens of square centimeters. Larger collecting areas lead to decreased optical efficiency (extracted concentrated optical power to incident optical power) due to propagation losses of the waveguide modes. Because of the limited area of a single LSC the coverage of significant areas in the range of square meters requires extensive electrical wiring which further increases the complexity and cost of such systems.